This invention relates to a color cathode ray tube and, in particular, to a main lens unit for electron guns which focuses and converges a plurality of electron beams.
In a color cathode ray tube, electron beams emitted from an electron gun assembly are focused and converged at a target where electron beam spots are formed. As one important factor for determining the performance of the color cathode ray tube, use is made, of, for example, the beam spot size on the target in which case the smaller the beam spot size the better. The beam spot size is determined by the performance of the electron gun assembly.
In general, the electron gun assembly comprises an electron beam generating section for effecting the generation and control of electron beams and a main lens unit for accelerating and focusing the electron beams. One effective method for improving the performance of the electron guns is to improve the performance of the main lens unit.
The main lens unit is mostly formed of electrostatic lenses and a plurality of electrodes having an opening are located on the same axis so as to apply a predetermined potential.
The electrostatic lenses are classified into various types by the differences in their electrode configuration. Basically, the lens performance can be improved by either increasing the diameter of the electrode opening to provide a larger-aperture lens or by making the electrode-to-electrode distance longer to provide a lens of a longer focal point.
However, since electron guns are inserted into a narrower glass cylinder section called the neck section of the color cathode ray tube, the electrode opening, i.e., the lens aperture, is physically restricted and, moreover, the electrode-to-electrode distance is restricted in order that a converging electric field created between the electrodes may not be influenced by the other undesired electric fields within the neck section.
In particular, with the three electron guns arranged in a delta array or in an in-line array as in the color cathode ray tube, the corresponding three electron beams are readily converged at one point in the neighborhood of a whole screen surface as the beam-to-beam distance becomes smaller and smaller. Furthermore, the deflection power may also advantageously be decreased. Thus the design tendency is toward decreasing the distance between the electrodes and toward decreasing the electrode opening.
Japanese Patent Publication (KOKOKU) No. 49-5591, corresponding U.S. Pat. No. 3,448,316 and U.S. Pat. No. 4,528,476 disclose a lens unit in which, in order to improve the lens performance, three electron lenses in a common plane are replaced by a single electron lens corresponding to a completely superimposed lens array so that three electronic beams pass through the center portion of that large-aperture electron lens in an intersecting fashion. FIG. 1 shows a view corresponding to an optically equivalent array of the electron guns as disclosed in Japanese Patent Publication (KOKOKU) No. 49-5591 (U.S. Pat. No. 3,448,316).
The electron beams 3A, 3B, and 3C emitted from cathode units 9A, 9B and 9C are previously focused by prefocusing lenses 20A, 20B and 20C and directed toward the center of larger-aperture electron lens 51. The electron beams 3A and 3C emerging from larger-aperture electron lens 51 are strongly deflected by deflecting means 51A and 51C at a deflection angle +.phi..degree. and converged at screen 30.
In this type of the tube, the center electron beam encounters no aberration while, side electron beams 3A and 3C experience a greater deflection error or coma so that the three electron beams 3A, 3B and 3C are formed, as spots of different sizes, on screen 30. In order to alleviate the beam deformation, control is required to somewhat weaken the deflection performance of larger-aperture electron lens 51 in which case it is not possible for larger-aperture lens 51 to exhibit its own inherent performance.
FIG. 2 shows an optically equivalent array of electron guns as disclosed in U.S. Pat. No. 4,528,476. As evident from FIG. 2, electron beams 3A, 3B and 3C emitted from cathode units 9A, 9B and 9C are preliminarily focused by prefocus lenses 20A, 20B and 20C and directed to the center of one larger-aperture electron lens 51 after one of the beams has been deflected by deflection lens 53A and another has been deflected by deflection lens 53C.
The electron beams 3A and 3C emerging from large-aperture electron lens 51 are strongly deflected, by larger-aperture lens, at a greater deflection angle and converged at screen 30. Even in this electron gun assembly, electron beams 3A and 3C encounter a greater deflection error or coma, so that three electron beams 3A, 3B and 3C emerge as beam spots of different sizes on screen 30.
As set out above, in U.S. Pat. No. 4,528,476, and Japanese Patent Publication No. 49-5591 (U.S. Pat. No. 3,448,316), larger-aperture converging lens 51 cannot fully exhibit its own inherent performance.
In order to prevent such a strong deflection, it may be considered that electron beams are incident in a parallel mode onto a larger-aperture converging lens such that the center electron beam passes through the lens with both the side electron beams passing through the marginal portions of the lens. Even in the electron gun assembly of this type, however, both the side beams are over-converged so that the three electron beams are separated too away from one another on the screen.
Thus, as has been indicated, the problems arise from the conventional structure where the three electron beams pass through the common larger-aperture lens and hence difficultly is encountered in putting it into actual use.